1. Field of the Invention
In the oil and gas industry there is frequently a need to fix within a spool or other tubular wellhead member a support ring which is capable of taking large axial loads. For example, such a ring may form part of a hanger suspension system for a tubing or casing string, and/or provide an axial bearing surface for a load set high pressure seal.
2. Description of the Related Art
One construction of ring which has been considered for this purpose is a split metallic ring, that is to say an almost continuous ring which is discontinuous and has a narrow gap at one angular position. The ring may be expanded and held expanded by a tool, such as one having a frusto-conical surface which moves axially through the ring. The ring has an external profile, such as a "knuckle" profile consisting of a series of axially spaced annular ribs, which, as the ring is expanded, enter complementary annular grooves in the surrounding wall of the tubular member. This provides a very secure fixing of the ring axially within the tubular member without making significant inroads into the wall thickness of the tubular member, or of providing within the tubular member a permanent landing bowl which would reduce the minimum diameter of the tubular member and hence limit the maximum size of casing strings or other components which could be introduced down through the tubular member before fitting of the load ring.
However, a disadvantage with the use of such expansible metal split load support ring is that, if it is to have sufficient radial thickness to provide the large bearing area which is often required, the ring cannot be expanded sufficiently to ensure secure locking with the surrounding tubular member, without the metal from which the ring is made yielding beyond its elastic limit and taking a permanent deformation. This is disadvantageous as it makes it difficult to recover the ring since it will not contract elastically when the expanding force is removed, and will remain in engagement with the tubular member. The problem is exacerbated in that such a metallic support ring will normally be made of suitable alloy to avoid corrosion by, for example, hydrogen sulphide emanating from the well, and such alloys have restrictive yield strengths.
It has previously been sought to overcome this problem by providing in the load support ring a series of angularly spaced radial cuts extending part-way through the radial thickness of the ring. However this significantly reduces the overall strength of the ring.